It is known that fuel cell stacks produce electrical energy directly via an electrochemical redox reaction using hydrogen (the fuel) and oxygen (the oxidant) without passing via a mechanical energy conversion step. This technology seems promising, especially for motor vehicle applications. A fuel cell stack comprises in general the series combination of unitary elements each consisting essentially of an anode and a cathode separated by a polymeric membrane allowing ions to pass from the anode to the cathode.
Insufficient supply of hydrogen (H2 starvation) is known to be a major cause of catalyst degradation in fuel cells of the PEFC type. Insufficient supply of hydrogen must absolutely be avoided during operation so as to prevent corrosion of the carbon generally used as catalyst support for PEFCs, but according to the inventor's experience it is also necessary to guarantee the presence of hydrogen at the anode during the phase of shutting down the fuel cell stack and while the fuel cell stack is at rest.
In general, a fuel cell stack is extinguished by extending the normal electrochemical reaction of the fuel cell stack until the voltage has collapsed due to complete consumption of at least one of the residual gases. To limit the abovementioned degradation mechanisms, it is necessary to ensure that the oxygen is exhausted before the hydrogen. It is therefore necessary to ensure that the fuel cell stack is supplied with hydrogen at the anode until complete extinction.
It is also necessary to guarantee the presence of hydrogen at the anode during rest, so as to maintain it at a 0 V RHE electrochemical potential (RHE being the abbreviation for reference hydrogen electrode). A voltage expressed in V RHE is therefore an electrochemical potential relative to that of hydrogen. Since pure hydrogen is not recommended for safety reasons, a hydrogen/nitrogen mixture is therefore recommended for the rest phases.
To guarantee the presence of hydrogen at the anode during the shut-down phases, patent application WO 06/012954 proposes purging the excess oxygen to atmosphere. However, to do this it is necessary for the cathode pressure to be high enough, something which may not always be guaranteed. Moreover, at the end of this procedure, during which air is naturally drawn in by the reduced pressure, the residual pressure in the cathode circuit is at most equal to the atmospheric pressure. By cooling, the pressure will tend to fall below atmospheric pressure, thereby increasing the migration of air into the fuel cell stack, the oxygen content of which will react with the residual hydrogen thus contributing to rapid hydrogen starvation.
Document U.S. Pat. No. 6,939,633 proposes a device for generating pressurized nitrogen from the ambient air. To do this, it is proposed to use a reactor in the cathode circuit in which the oxygen of the air introduced into the fuel cell stack by means of a pressure source is made to react with hydrogen coming from the main tank. However, no arrangement is provided for preventing hydrogen starvation. On the contrary, the proposed procedure provides for the fuel cell stack to be left with air present at the anode and at the cathode (column 8, lines 29-31). Moreover, this solution is complicated and requires the addition of a catalytic reactor in the cathode circuit. Even though the possibility of using the catalyst already present in the fuel cell stack is mentioned (column 7, lines 59-64), in both cases the hydrogen and the oxygen of the air react directly on a catalyst generating no electricity, only heat. Finally, this solution requires two communications between the anode circuit and the cathode circuit, a first valve (344) for introducing hydrogen to the cathode, so as to react with the oxygen of the air, and a second valve (346) for inundating the anode with the nitrogen generated at the cathode, thereby compromising safety in the event of one of these valves failing (inopportune opening or leakage).
Patent application US 2009/0220832 proposes a fuel cell stack comprising a hydrogen buffer tank, a recirculation loop to the cathode, and valves for isolating the internal circuits of the stack from the atmospheric air. This application demonstrates the importance of preventing hydrogen starvation during extinction and during the rest period that follows, so as to prevent long-term electrode oxidation. However, the extinction procedure described transiently involves hydrogen/oxygen mixtures, thereby compromising safety. Moreover, the proposed arrangement of the components and the procedure described are intended to inundate the circuits of the stack with practically pure hydrogen rather than with a nitrogen-hydrogen mixture, which is neither safe nor economic.